feat(chart): layers panel + selection-first fib

- Adds Layers drawer (visible/lock/opacity) and effective opacity wiring.
- Implements selection-first overlay: click selects, drag moves selected, Ctrl-drag edits B, Space-drag always pans.
- Routes pointer events via capture and adds Fib opacity support.
- Updates .gitignore to keep secrets/local scratch out of git.

.gitignore

@@ -1,7 +1,14 @@
+# Secrets (never commit)
+tokens/*
+!tokens/*.example.json
+!tokens/*.example.yml
+!tokens/*.example.yaml
+
 node_modules/
 dist/
 .env
 *.log
-tokens/*.json
-tokens/*.yml
-tokens/*.yaml
+
+# Local scratch / build output
+_tmp/
+apps/visualizer/dist/